1. Field of Invention
The present invention relates to a time adjustment device that corrects the time based on signals from a positioning information satellite such as a GPS satellite, to a timekeeping device that has the time adjustment device, and to a time adjustment method.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on known orbits, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time) with extremely high precision.
Japanese Unexamined Patent Appl. Pub. JP-A-H11-211858 teaches a radio-controlled timepiece that analyzes the time code contained in a long-wave standard time signal to correct the displayed time instead of using GPS satellite signals or a method of correcting the time based on GPS time information.
The time information transmitted in a GPS satellite signal is updated on a predetermined cycle. Japanese Unexamined Patent Appl. Pub. JP-A-H11-125666 teaches technology for predicting the GPS time information after being updated at this predetermined period, predicting the time of the next GPS time signal, and using this predicted time to acquire the positioning information for the device location. Measuring the pseudo range to the GPS satellite and determining the current position is therefore possible even when the reception environment is not ideal.
Japanese Unexamined Patent Appl. Pub. JP-A-H10-82875 teaches a method of correcting the time using the time information (GPS time) from a GPS satellite.
This method acquires the navigation message at full power (that is, with the CPU running and other parts operating) immediately after the power is turned on. The time information contained in the acquired navigation message is then acquired and the time is calculated. The time is then calculated and the timing for the next correction is determined from the relationship between the precision of the crystal that generates the reference clock signal of the device and the required precision of the timepiece. More specifically, the time when the next navigation message will be acquired (when the CPU is stopped and a sleep mode is active) is determined. The navigation message is then acquired again after the sleep mode ends, and the time is corrected based on the time information acquired from the navigation message.
With this method the receiving device determines when to receive the GPS signal, such as immediately after the power turns on. The user, however, might also want to force adjusting the time based on the received GPS time. In such cases the reception time must be adjusted so that the GPS time can be received and the time can be adjusted at a time close to when the user wants to adjust the time. However, because minimizing power consumption is essential in a timepiece or other small device, it is also essential to acquire the information needed to set the time in the shortest time possible even when satellite signals are received from a GPS satellite or other positioning information satellite to adjust the time at a timing close to when the user wants to adjust the time.